This tutorial shows how to modify and minimize small servos. 3.7g and 2.5g servos are very handy in small designs because they are cheap, small, and have built in controller and gearbox. However to use them in continuous rotation mode some hacking is necessary. I'm using these servos on my nano sumo robot so I also have to modify servos to be smaller.

Comparison of 2.5g and 3.7g servos.

Picking your servo:

There are two kinds of small servos that interest me: 2.5g and 3.7g. These are the smallest rotary servos I could find from internet. Both of them are easily modifiable and easy to use. Differences between them are:

bigger one is cheaper

smaller one has weird connector

bigger one has extra support ring on output shaft (eq. longer output shaft)

bigger one has four metal screws, smaller has none

smaller one has 1mm shorter body

So they have minor cosmetic differences but the insides is exactly the same - same motor, same gearbox, same electronics.

3.7g servo measurements

So getting it to continuous rotation is the same as with other servos: Pop out the potentiometer and solder two 2.2k resistors on its pads. Now to the hard part - making it small.

My goal was to fit two servos and wheels to nano sumo size (25x25x25mm). Also I wanted that my wheels were on same axis. After lots of measuring and drawing I came to conclusion that the only way to accomplish my goal was to make the gearbox shorter. So, lets do it!

Cut away half of the middle plastic piece. I recommend to use dremel with cutting wheel on low speeds. To understand what to saw look at the picture.

Cut away two little ears on servo.

After that cut off the excess white shaft so the motor will be L shaped. Now you can rig the electronics and clue two of these together to get differential robot mechanics. This kind of motors were used in Kobold 1.0 mechanics.

Or, you can continue on the next step.

Modding the gearbox:

Gearbox: motor, plastic case and gears

Dissassemble the gearbox.

Take old gear away from motor

Take plastic base, find axis hole that was nearest to motor and drill it to 3mm.

Find single plastic gear, the one on the right on the picture.

Put motor through the hole and glue the gear on the motor, smaller gear closer to motor center.

How to assemble the motor.

Then put the other gears in their right place (reference the picture above)

Finished gearboxes and some wheels

Modify the top case as you like (I cutted off the sides) and put it back to its place.

Press motor closer to other gears, into your newly drilled hole. The idea is to make the gears to fit.

If you are really sure the gears are fine then glue the motor in the place.

Electronics:

Electronics with some extra resistors soldered on it

Electronics need hacking too. You need to unsolder the potentiometer leads and solder two 2.2k resistors in place. The value of the resistors is important, servos I used had 4.7k pots and according to servo driver ic chip I can't solder whatever to there.

Both resistors connect to pad where the pots middle lead was connected. On my board it was unfortunately the left one. For maximum smallness I soldered one resistor in 0603 package from center pad to left one and other one standing on right one. Then I used piece of pin to connect other end of resistor to left pad. Its ugly but small and most important of all - works.

Wheels:

Wheel with servo horn

And because this is after all Kobold mechanics tutorial then I have to include how to make Kobold compilable wheels. First, take M5 wide washer (DIN 9021) and a random servo horn. then press them together. Then apply some glue and cut off everything you don't need. Yai! Now more pictures.

Finished second generation mechanics! And yes, thats a uC as a roller..